Field of the Invention
The present invention relates to an image processing apparatus, a method of controlling the image processing apparatus, and a storage medium and, more particularly, to a technique of processing a medical image captured by a medical image acquisition apparatus (modality) such as an X-ray computed tomography apparatus (X-ray CT).
Description of the Related Art
In image diagnosis using a medical image (for example, a three-dimensional tomographic image representing information inside an object), image capturing is sometimes performed by using a contrast material to emphasize an anatomical portion such as a lesion portion, blood vessel, or specific organ. Administering a contrast material will increase the contrast between adjacent organs or of a lesion portion, blood vessel, or the like existing in an organ, thereby facilitating the grasping its shape. However, at different elapsed times since the administration of a contrast material, even the same anatomical portion exhibits different degrees of contrast enhancement. For this reason, information identifying the elapsed time since administration is recorded on an image, medical record, or the like.
For example, in contrast material-enhancement CT, a parameter called a phase is defined, which represents that image capturing has been performed after a specific time elapsed since the administration of a contrast material. This parameter is included in the header portion of an image. Phases are often classified into four categories when they are recorded. The first category is “non-contrast enhancement” indicating that image capturing has been performed without administering any contrast material. With regards to images captured by administering a contrast material, phases are classified into the following three categories: “early phase”, “portal phase”, and “late phase” in ascending order of elapsed time since contrast material administration.
The gray level distribution of an anatomical portion such as an organ greatly changes with phase even within the same portion. In the image processing of extracting, for example, the region of each portion, when processing images in various phases, it is sometimes impossible to perform accurate extraction with one image processing parameter. There is available the MPA (Maximum a Posteriori) method as one of typical region extraction methods for medical image processing. This method calculates a likelihood indicating that each voxel in an image belongs to a specific portion by using a probabilistic atlas prepared in advance, which represents the existence probability of each portion, and the prior distribution of the gray levels in the respective portions. The region of each portion is then extracted from an image based on the likelihood.
According to Hyunjin Park, Peyton H. Bland, Charles R. Meyer “Construction of an Abdominal Probabilistic Atlas and its Application in Segmentation”, IEEE Transactions on Medical Imaging, Vol. 22, No. 4, April 2003, liver and kidney regions are extracted by using the MAP method. It is important for the MPA method to provide the prior distribution of the gray levels in each portion with as high accuracy as possible.
However, according to the technique disclosed in Hyunjin Park, Peyton H. Bland, Charles R. Meyer “Construction of an Abdominal Probabilistic Atlas and its Application in Segmentation”, IEEE Transactions on Medical Imaging, Vol. 22, No. 4, April 2003, since the gray level distribution of each portion greatly changes with phase, when the phase information of image data is unknown, it is impossible to accurately provide a prior distribution of gray levels and hence to stably perform region extraction. In addition, any deficit or error in phase information in image data will disable the selection of a proper image processing parameter using phase information (for example, the selection of the prior distribution of the gray levels in each portion in the MAP method). That is, the problem is that any deficit or error in phase information in image data will decrease the accuracy of subsequent image processing.
The present invention has been made in consideration of the above problem and provides a technique of improving the accuracy of subsequent image processing even with a deficit or error in phase information in image data.